Add start_date to financing plans and protect paid paychecks from refresh

- Migration 004: adds start_date column to financing_plans (DEFAULT CURRENT_DATE)
- generatePaychecks: skips financing plans whose start_date is after the target month
- buildVirtualPaychecks: same start_date guard for virtual previews (already applied)
- generatePaychecks upsert: preserves gross/net when paycheck has any paid bills
- financing.js POST/PUT: accept and store start_date
- Financing.jsx: add Start Date field to create/edit form (defaults to today)
- CLAUDE.md: document start_date guard and paid-paycheck refresh protection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/routes/paychecks.js

@@ -96,6 +96,12 @@ async function buildVirtualPaychecks(year, month) {
   const client = await pool.connect();
   try {
     for (const plan of activePlans.rows) {
+      // Skip plans that haven't started yet for this period
+      const start = new Date(plan.start_date);
+      const planStartYear  = start.getFullYear();
+      const planStartMonth = start.getMonth() + 1;
+      if (year * 12 + month < planStartYear * 12 + planStartMonth) continue;
+
       const amount = await calcPaymentAmount(client, plan, year, month);
       if (amount <= 0) continue;
 
@@ -155,8 +161,20 @@ async function generatePaychecks(year, month) {
          VALUES ($1, $2, $3, $4, $5, $6)
          ON CONFLICT (period_year, period_month, paycheck_number)
          DO UPDATE SET pay_date = EXCLUDED.pay_date,
-                       gross = EXCLUDED.gross,
-                       net = EXCLUDED.net
+                       gross = CASE
+                         WHEN EXISTS (
+                           SELECT 1 FROM paycheck_bills pb
+                           WHERE pb.paycheck_id = paychecks.id AND pb.paid = TRUE
+                         ) THEN paychecks.gross
+                         ELSE EXCLUDED.gross
+                       END,
+                       net = CASE
+                         WHEN EXISTS (
+                           SELECT 1 FROM paycheck_bills pb
+                           WHERE pb.paycheck_id = paychecks.id AND pb.paid = TRUE
+                         ) THEN paychecks.net
+                         ELSE EXCLUDED.net
+                       END
          RETURNING id`,
         [year, month, num, payDate, gross || 0, net || 0]
       );
@@ -183,6 +201,12 @@ async function generatePaychecks(year, month) {
       `SELECT * FROM financing_plans WHERE active = TRUE ORDER BY due_date ASC`
     );
     for (const plan of activePlans.rows) {
+      // Skip plans that haven't started yet for this period
+      const start = new Date(plan.start_date);
+      const planStartYear  = start.getFullYear();
+      const planStartMonth = start.getMonth() + 1;
+      if (year * 12 + month < planStartYear * 12 + planStartMonth) continue;
+
       // Determine which paycheck(s) this plan applies to
       const targets = plan.assigned_paycheck == null ? [1, 2] : [plan.assigned_paycheck];
       for (const pcNum of targets) {